Light-emitting diodes (LEDs) are semiconductor devices made of a compound semiconductor material containing III-V group elements, for example, GaN, GaP, GaAs, and the like. The lifespan of the LED is up to 100,000 hours, and has advantages of quick response speed (approximately 10−9 seconds), small volume, power-saving, low pollution, high reliability, and ease mass production. Thus, the LEDs have been intensively used in many fields, for example, illumination device, traffic lights, cellular phones, scanners, fax machines, etc.
Currently, sapphire (Al2O3) substrates are often used in GaN-based LEDs. Thermal conductivity of sapphire substrates is concerned when GaN based LEDs are fabricated over the sapphire substrates. Accordingly, silicon substrates with better thermal conductivity are gradually used in fabrication of GaN-based LEDs. In addition to good thermal conductivity, the silicon substrates have many advantages, such as high electrical conduction, large wafer size and low cost.
The semiconductor device having the silicon substrate faces issues that the lattices and the thermal expansion coefficients (CTE) mismatch between a first doped semiconductor layer (i.e. GaN-based III-V compound) and the silicon substrate. The semiconductor device suffers a stress resulted from the mismatches of the lattices and the thermal expansion coefficients. Due to the stress, the semiconductor device bends severely and possibility of crack increases. Therefore, it is a great challenge in reducing the crack possibility.